These cast irons have high abrasion-resistance, and can be readily cast in shapes required in machinery used for crushing and grinding, etc. Abrasion-resistance increases with the increase of hardness due to the increase of the volume of the eutectic carbides (alloy eutectic carbides are much harder than iron carbide). The must element chromium prevents the formation of graphite during solidification and ensures the stability of carbides.
Ni, Mo, Cu or a combination of these elements prevent the formation of pearlite by increasing the hardenability. Low-alloy pearlitic white irons have hardness between 350-550 BHN, whereas high-alloy white irons have between 450-800 BHN.
High-alloy white cast irons normally have austenite matrix in as-cast state, but is changed to hard martensite by heat-treatment. Carbon content is high when maximum abrasion-resistance is desired, but is on lower side, when repeated impact loading is of primary importance in service.
High-alloy white cast irons are divided into the following groups:
ADVERTISEMENTS:
1. Ni-Hard Type I to IV: Nickel-Chromium White Irons:
These have 3-5% Ni, 1-4% Cr (Ni-hard IV has 7-11% Cr). Ni-hard IV with 3.2% C, 9% Cr, 6% Ni has 410 BHN hardness, 690 MPa, compressive yield strength, 160 J unnotched impact energy in the austenite state; 750 BHN hardness, 2450 MPa compressive yield strength, 170 J unnotched impact energy in martensite state.
2. Chromium-Molybdenum White Irons:
These have 11-23% Cr, upto 3% Mo, and also have some Ni or Cu. An alloy (2.9% C, 18% Cr, 2% Mo, 1% Cu) has hardness of 510 BHN, 1100 MPa compressive yield strength, 190 J unnotched impact energy in austenite state; hardness of 730 BHN, 2620 MPa compressive yield strength, 200 J unnotched impact energy in martensite state.
ADVERTISEMENTS:
3. High Chromium White Irons:
These have 25% or 28% Cr, and other elements such as Mo and/or Ni up to 1.5%. An alloy (3.0% C, 27% Cr) has hardness of 600 BHN, 1380 MPa compressive yield strength, 170 J unnotched impact energy in austenite state; hardness of 680 BHN, 1460 MPa compressive yield strength, 175 J unnotched impact energy in martensite state.
1. Ni-Hard- Nickel-Chromium White Irons:
These martensitic white irons are very cost-effective for crushing and grinding components. The composition is chosen depending whether abrasion resistance is the prime requirement, or repeated impact loading. When maximum abrasion-resistance is essential, carbon is kept between 3.2 to 3.6%, such as in Ni-hard I. For better repeated impact resistance, a carbon between 2.7-3.2% gives lesser carbides to result in greater toughness such as Ni-Hard II.
ADVERTISEMENTS:
Ni-hard III is designed for chill-casting of grinding balls and slugs. Ni-Hard IV has high hardness, but the microstructure has discontinuous eutectic carbide distribution of M7C3, chromium carbides, which also have appreciable resistance to fracture by impact (due to discontinuity).
The improved corrosion resistance of the alloy makes it useful in handling of corrosive slurries. Nickel increases hardenability to suppress transformation of austenite to pearlite, and its content is increased with thickness of the casting.
For 1.5 to 2 inch thickness 3.4 to 4.2% Ni; for heavier sections, Ni up to 5.5% is added to avoid pearlite formation on mould cooling; but the increased Ni increases the amount of retained austenite which results in lower hardness. A minimum amount of silicon is needed to increase the fluidity of the melt and to produce a fluid slag. 1-1.5% Si increases the amount of martensite formed, and thus, the hardness. Late additions of 0.2% ferrosilicon (75% Si) improve the toughness.
High silicon promotes pearlite formation, and thus higher amount of nickel is needed to counter this effect. Chromium promotes carbide formation countering the effects of nickel and silicon in Ni-Hard I, II and III, in amounts 1.1 to 4.0%, as the content is increased with increasing section-thickness. Ni-hard IV has around 9% Cr to form discontinuous carbides, (M7C3 type), which are harder with improved toughness.
ADVERTISEMENTS:
Manganese is kept a maximum of 0.8%. Although, it increases hardenability to avoid pearlite formation, but being austenite stabliser, increases the amount of retained austenite, and thus lowers the as-cast hardness. Copper increases hardenability but like Mn increases retained austenite. Copper is a nickel substitute; a judicious copper content reduces the expensive nickel requirement. Molybdenum increases hardenability to inhibit pearlite formation, specially in heavy-section castings.
Heat Treatment of Ni-Hard White Irons:
To avoid thermal-shock cracking, no white iron casting (being brittle) be placed in hot furnace or rapidly heated or cooled, especially the complex shapes.
ADVERTISEMENTS:
i. Stress-Relieving Treatment (Tempering):
As the as-cast matrix structure is martensite plus some retained-austenite, some of the stresses induced by the transformation (from austenite) are relieved by heating to 205-260°C for at least 4 hours. The treatment also tempers and increases the strength and the impact toughness by 50 to 80%, without reducing the hardness, or abrasion-resistance. Some retained- austenite may transform to martensite while cooling from the tempering/stress-relieving temperature.
ii. High Temperature Treatment:
When the as-cast hardness is not sufficient, then, this treatment is given to castings. It consists of heating at a temperature 750-790°C for 8 hours, followed by air or furnace cooling at a rate not more than 30°C/h. Tempering may be done at 205-260°C, but sub-zero treatment is able to achieve a hardness of 550 BHN, if 60% martensite is present in the matrix; a hardness of 650 BHN is attained when martensite is 80-90%.
To reduce the retained-austenite by transforming it to martensite, the castings (after austenitisation) are refrigerated to -70 to -180°C for half to one hour. Martensite, retained austenite and carbides in Ni-hard IV alloy after deep-freezing. Tempering at 205-260°C is then done.
2. Chromium-Molybdenum and High Chromium White Irons:
Chromium-molybdenum white irons contain 11-23% Cr, and upto 3% Mo. Molybdenum (Ni, Cu if needed) prevents pearlite formation. These are the hardest of all grades of white irons because of the harder eutectic carbides, and the matrix is treated to form hard martensite to achieve in castings high hardness, and thus maximum abrasion resistance and reasonable toughness. This requires heat treatment involving air-quenching from high temperature, following by tempering at 200°C.
High chromium (25% Cr and 28% Cr) white irons have the best combination of excellent abrasion resistance and toughness (to bear the repeated heavy impact loading), because these have hard but discontinuous M7C3 eutectic carbides (low Cr-alloys have relatively softer and continuous M3C eutectic carbides).
To prevent pearlite formation, and to get maximum hardness, Mo up to 1.5% is added in all but the lightest cast-sections. Hardness is not that high as in chromium-molybdenum alloys, but these alloys also have good corrosion-resistance. Thus, these alloys find applications in slurry pumps, brick-moulds, coal grinding mills, rolling-mill rolls, shot-blasting equipment, parts for quarrying, hard-rock mining and milling.
High chromium irons (26-28% Cr, 1.6-2% C) are low carbon irons with improved corrosion resistance for use as pumps handling fly-ash. The resistance to chloride containing atmospheres is improved by adding 2% Mo and by having complete austenitic matrix (as cast), which results in some reduction in abrasion- resistance.
Because of castability and cost (much cheaper than stainless steels), high chromium (12-39% Cr) white iron castings of complex and intricate shapes are used for high temperature applications. Chromium helps to form an adherent, complex, chromium-rich oxide film which resists scaling upto 1040°C.
Depending on the matrix, high chromium irons fall in three groups (carbon varies between 1-2%):
1. Martensitic matrix (12-28% Cr) irons
2. Ferritic irons (30-34% Cr)
3. Austenitic irons (15-30% Cr, 10-15% Ni to stabilise austenite)
The ferritic alloys are used as recuperator tubes, breaker-bars, and trays in sinter furnaces, grates, burner nozzles, other furnace parts; glass-bottle moulds, valve-seats for combustion engine.
Importance of Microstructures:
High chromium white irons should have good wear resistance (that is hardness) and good toughness to increase the tolerance to repeated impact loading. Extreme of each is possible at the expense of the other. The microstructure normally consists of very hard, wear resistant but brittle carbides (continuous or discontinuous) along with metallic matrix.
In general, the wear resistance increases with increase of hardness of carbides (M7C3 chromium carbides are harder than M3C eutectic carbides) and by increasing the amount of carbides (by increasing the carbon content). Whereas the toughness improves by, increasing the proportion of metallic matrix (reducing the carbon content) and by having discontinuous carbides.
Large hexagonal carbide (primary carbides) rods are formed when the carbon content exceeds the eutectic carbon (eutectic carbon uniformly decreases from around 3.8% C at 10% Cr to 2.8% at 28% Cr), and such carbides are avoided in a casting subjected to any impact in service, because these drastically reduce impact toughness. Normally, castings thus are hypoeutectic in structure.
The metallic matrix can be pearlite, austenite or martensite, or some combination of these, when the casting is put in service. Pearlitic matrix has modest wear and abrasion-resistance with low toughness and thus, is avoided. As cast austenitic matrix, is soft to give satisfactory performance in some abrasive wear applications because it gets work-hardened in service.
Also, fully austenite matrix has greatest resistance to crack propagation. A tempered martensitic matrix has maximum resistance to spalling under conditions of repeated impact alongwith good wear resistance. Thus, tempered martensite provides good wear-resistance and reasonable fracture-toughness.
Martensitic matrix can be obtained in as-cast castings if sufficient alloying elements are present to suppress pearlite formation during its cooling. But the presence of large amount of retained austenite reduces hardness to much lower levels than achieved in heat-treated martensitic structure. Subcritical annealing is used to reduce this austenite content.
Many complex castings such as slurry pump components are often used in the as-cast austenitic/martensitic state to avoid the risk of cracking and distortion during heat treatment. Optimum performance is usually achieved with heat-treated martensitic matrix obtained by air-quenching from high temperature, followed by tempering at 200°C.
For this heat-treatment, the desired mould-cooled casting-microstructure is often pearlite. This shortens the response to heating in heat treatment, apart from facilitating the removal of gates and risers with little induced stresses.
The amount of alloying elements should be enough to assure substantial pearlite in mould-cooled structure, but provides enough hardenability to prevent pearlite formation during air-quenching after austenitisation in the heat-treatment cycle.
Heat Treatment of Cr-Mo and High-Cr White Irons:
The main heat treatment is hardening (austenitising, air-quenching) and tempering to induce maximum abrasion-resistance with reasonable fracture-toughness. Annealing, stress relieving are also given.
Hardening and Tempering:
Depending on the thickness of the casting, there should be sufficient alloying so that air-quenching from the austenitization temperature results in martensite, i.e., the cast iron has sufficient hardenability to allow air-hardening, i.e. pearlite free micro- structure. Table 15.20 gives minimum alloy content for different thickness (or radius) to avoid pearlite formation. Faster cooling is avoided to risk cracks and distortion due to thermal as well as or transformation stresses.
Over-alloying with Mn, Ni, Cu, results in large amount of retained austenite to decrease the resistance to wear and spalling. As high chromium irons are quite brittle, the castings are heated slowly in a cold furnace to 650°C to avoid thermal cracking (as these are bad conductors of heat). Complex-castings are allowed a maximum heating rate of 30°C/h, although simple shapes and fully-pearlitic castings can be heated faster.
The heating rate above 650°C can be increased as illustrated in Fig. 15.26:
An optimum austenitizing temperature although, results in, maximum hardness, but it varies with the composition of the iron. This temperature controls the dissolved carbon in austenite. Higher temperature increases the stability of austenite, which results in larger retained-austenite (after quenching), to reduce the hardness and wear resistance of iron.
While low temperature results in, lesser dissolved carbon in austenite which produces low carbon martensite resulting in reduced hardness and wear resistance. (A good heat-treatment should produce, on air-cooling, precipitation of line secondary M7C3 carbides within the austenite to destablize the austenite).
Austenitising temperatures are:
Castings with thick-sections are austenitized at higher temperatures within the range.
The soaking time is 4 hours minimum, or 1 hour per 25 mm for thick castings. It is essential to get equilibrium dissolution of chromium or other carbides to make the iron air-hardening. Holding time is lesser for fully-pearlitic structure.
The austenitized castings are air-quenched (vigorous fan- cooling) to below the pearlite transformation temperature range of 550- 600°C, and then, the subsequent cooling rate is decreased by cooling in air or in furnace to room temperature in order to minimise the stresses developed during martensitic transformation. Complex shapes or thick- castings are put in a furnace maintained at 550-600°C, and soaked for time to get uniform temperature within the casting.
The castings, then, may be furnace-cooled, or still-air-cooled to room temperature. It is better to minimize the development of martensitic transformational stresses by slow and controlled cooling (by minimizing temperature gradients and differential transformation in the casting) through the martensitic transformation temperature range (≤ 260°C), because the stress-relieving treatment at 540°C later, if given, results in substantially reduced abrasion-resistance.
The castings at this stage contain 10-30% of retained-austenite. Although some castings may be put in service in this state, but normally tempering or subcritical heat treatment is given. Tempering is done by reheating the castings to 200-230°C and soaking there for 2-4 hours to substantially improve (~ 20%) fracture-toughness, and also to relieve residual-stresses.
Subcritical Treatment:
Large-sized air-quenched martensitic castings may be given subcritical treatment in order to eliminate the retained-austenite, resulting thus, in the increase of resistance to spalling.
The treatment consists of heating the castings to 480-540°C for 8-12 hours, but the temperature and the time at the temperature have important bearings on properties. Excess time or temperature may soften the castings with drastic reduction in the abrasion resistance; whereas, insufficient temperature does not eliminate the austenite completely.
Annealing:
Subcritical-annealing or full-annealing may be done to make the castings more machinable. Subcritical annealing is the process of heating in the range 690-705°C, and soaking for 4 to 12 hours, resulting in a hardness of 400-450 BHN. Full-annealing results in still lower hardness, when the castings are heated in the range of 950-1010°C, followed by slow cooling to 760°C; soaking at this temperature for 10 to 50 hours, depending on the composition. After machining, the castings may be hardened and tempered.